A combination of two immunotherapy drugs used against highly advanced melanoma — the deadliest form of skin cancer — caused tumors to dramatically shrink or disappear, according to a study released Wednesday. The results of the research from the Memorial Sloan Kettering Cancer Center in New York are part of a transformative advance in harnessing the body's immune system to kill cancer cells.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

In the new study, some 40 percent of patients in the study saw reduction in their huge tumors from the treatment, which combines a drug called ipilimumab and an experimental new immunotherapy known as Nivolumab. They're part of a treatment trend that uses the immune system to help reject cancer.

“I’m tremendously excited,” oncologist Dr. Jedd Wolchok told NBC News. “Having a combination of immunotherapies demonstrate this degree of regression in the number of patients and doing it quickly is something I have not seen before.”

In other studies released Wednesday, in advance of the annual meeting of the American Society of Clinical Oncology, immunotherapies are being found effective against lung, kidney and many other cancers, as well.

Immunotherapy has been a dream of cancer researchers for more than a century. But the field has been marked by a few tantalizing successes, followed by years of failure. Researchers believe they now understand the immune system well to start employing it reliably against cancer.

The treatment starts with the understanding that a type of immune system cells — white blood cells called killer T-cells — try to destroy cancer cells. But cancer cells are tricky and they release an array of chemicals to keep the killer T-cells at bay. To completely destroy the cancer cell, the killer T-cell usually has to connect to it in two places.

The first big break in immunotherapy was a drug approved to treat melanoma in 2011 called ipilimumab, marketed under the brand name Yervoy. It targets one of two sites where T-cells must attach to the cancer cells. That significantly helped about 11 percent of advanced melanoma patients, a number far below what anyone wants, but far higher than anything available before.

The next step was to aim the killer T-cells at the second site. Several experimental drugs have been developed for that. Wolchok and his team combined Yervoy with nivolumab (both are made by Bristol-Myers Squibb) — the two drugs together brought the response rate up to 40 percent. This was an early study, Phase I, usually used simply to determine the maximum dosage that does not cause side effects. Wolchok believes that at the optimal dose the response rate will be more like 65 percent.

Many companies are developing drugs against these immune targets and it is possible that other combinations will work even better. Their effectiveness does not seem to be limited to melanoma; early results show many cancers could benefit.

There are caveats.

All these studies are still in there early phases. Wolchok enrolled 86 patients for the latest melanoma trial. Much bigger studies will be required.

Also, the drugs have side effects — often nothing more than a bad rash, but they also can cause inflammation of the intestine or the liver. And they are expensive. Bristol-Myers Squibb charges $120,000 for the typical four-dose treatment with the only approved drug, Yervoy. When doctors start using combinations the costs can only multiply.

Despite those limitations, immunotherapy is an exciting new concept. Experts say it stands a very good chance of curing many patients whose cancer would have been fatal.

Video: Immunotherapy targets cancer cells with remarkable results

>>>as we mentioned, important, very encouraging news in the fight against cancer. specifically here, melanoma. far too common and it can be far too deadly when it is not caught in time. a new treatment approach is getting excellent results we are told and could be used to fight different cancers. we get our report on this tonight from our chief science correspondent
robert bazell
.

>>there it is. nicely done.

>> reporter: mary
elizabeth williams
is 47. in
2010
she got a diagnosis of stage four melanoma that had spread through her body.

>>it was the beginning of the school year for my kids. suddenly i didn't know if i was going to be there for the end of the school year.

>> reporter: williams got a new therapy that worked so well in a few months she got the best possible news.

>>he told me that my cancer was completely gone. i was so stunned and surprised.

>> reporter: unlike chemotherapy which directly kills
cancer cells
, immunotherapy works on killer t cells that devour bacteria and viruses.
cancer cells
put up a chemical shield to protect against t cells. the therapy breaks down that shield and the cells go to work. yervoy helps 10% of melanoma studies. doctors combined wit a second drug that hits a different target on t cells allowing them to kill more cancer.

>>i'm tremendously excited. we are seeing more than half of patients having significant regressions of their advanced melanoma.

>>how are you doing?

>> reporter: it's not just melanoma. because of similar drugs david gobin has seen his
lung cancer
disappear.

>>i can do a lot of things i couldn't before.

>> reporter: and ken kirkwood's cancer is gone. some believes this is one of the most -- curing patients in a new way. many similar experimental drugs are being tested and other combinations could work better. the drugs have
side effects
and will be very expensive there is no doubt they will save many lives.
robert bazell
, nbc news, new york.